Binding assays such as immunoassays, are in widespread use in clinical laboratories for the detection of substances in biological fluids. There is however increasing interest in the development of assays which can be performed without the need for complex analytical techniques and equipment, for example, by a physician in his consulting room or by a patient at home. Such assays are not only more convenient but allow savings in time and expense. Particular applications for which convenient and simple assays and reagent formulations are being sought are the detection of pregnancy and of the fertile period of the menstrual cycle.
It is known to conduct binding assays on a strip of material provided with a plurality of reagent zones, in which a developing solution forms a solvent front which passes along the strip by capillary action picking up and facilitating reaction between a sample and assay reagents located at the reagent zones (see for example, British patent specification GB-B-1589234). A feature of such strips is the existence of a test location at which, under certain conditions determined by the assay protocol and the sample composition, a labelled reagent becomes immobilised, giving an indication of the assay result. In early assays, the labelled reagent was a binding partner or analogue of the analyte to be measured, labelled with a radioactive isotope. Such assays require instrumentation to detect the level of radioactive label and may present health risk problems. A solution to this has been the use of enzyme labels which produce a characteristic signal (such as a colorimetric signal) with an appropriate substrate.
A significant problem in the design of such so called "dipstick" enzyme-labelled binding assays is the application of the appropriate enzyme substrate in order to produce a detectable signal. The signal may be developed by adding substrate to the appropriate position on the reagent strip after allowing the assay to proceed to completion. Alternatively, the appropriate part of the strip may be removed and chemically analysed. All of these represent steps which would be at least inconvenient, if not impossible for home use of the assay.